Dispensers of the aforementioned type are known from US2003/0169046 A1.
This document discloses a sheet (paper sheet) sensing means in the form of two sets of sensors (pairs of IR emitters and receivers) in the discharge chute of the dispenser to protect it from ambient infrared (IR), which sensors can detect a leading edge of a paper sheet to be dispensed and then dispense paper as required when a user is present. In a so-called “hanging towel” mode (“sheet hanging” mode), sheet material may be dispensed when absence of material is detected, as this indicates that a towel has been torn off. In both situations, the sensors register the position of a piece of sheet material after the feed mechanism starts to operate so that a leading edge is detected during a first predetermined time period. After detection, a predetermined further amount of material can be dispensed during a second predetermined period. At the end of the feeding cycle which lasts for the predetermined second time interval, a towel length of the required length will have been dispensed for grasping and tearing by a user. When a towel of predetermined length is irregularly torn, one of the sensors may be uncovered while the other one is covered, in which case the control system detects a torn state and allows a new towel to be issued on the next detection of a user.
While the aforementioned dispenser thus provides means for detecting an irregularly torn sheet, it however relies on the fact that a sheet is torn off irregularly at or after the intended time for being torn off, namely after the dispensing operation by the motor has finished. It also relies on the fact that, at that time (after motor feeding has stopped) at least one of the sensors will then still be uncovered.
However it has been recognised that an impatient user may tear off a sheet while it is being fed, so that when the remainder of the predetermined length of sheet (which has not been torn off) continues to be fed out of the discharge chute, the remaining part will cover both sensors. In the aforementioned device this circumstance would of course leave a quantity of sheet still present at the outlet and thus detected by both sensors, causing the system to register that a towel has not been torn off. This can prevent dispensing of a new piece of paper towel until the piece blocking the sensors is removed. Furthermore, as the sensors are in the discharge chute which is designed not to allow access by human fingers, the dispenser may remain inoperable due to the premature tearing that occurred, since no further sensors are located outside the discharge chute to determine that a paper sheet of sufficient length is not present.